When a DBMS that claims it is relational and yet it supports SQL's "features" that departs from the relational model, then that DBMS is not relational. If a DB

When a DBMS that claims it is relational and yet it supports SQL's "features" that departs from the relational model, then that DBMS is not relational. If a DBMS is "slow" or "fast", that is not because of SQL -- it is because of the specific implementation decisions that the makers of that DBMS took. And that has nothing to do with what you just said.